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Well wait a minute...I thought I saw somewhere on the FN303 information page that the rounds are spun to increase accuracy and distance???

The next question would be that if the barrel wasn't implementing the spin then what was? What was causing the rounds to behave the way they were? I mean, if it were just a matter of weight and no spin was involved then wouldn't a heavier paintball show the same accuracy as the FN303 rounds since we are seeing these results with the lighter rounds? These results would seem to lead to the conclusion that there are forces acting on the ball that are greater than the vortexes we see on a round ball thus pushing the lighter round further off course but not affecting the heavier round.

It sounds to me like there may be something else at work here. hmmmmmmmm.

Well wait a minute...I thought I saw somewhere on the FN303 information page that the rounds are spun to increase accuracy and distance???

The next question would be that if the barrel wasn't implementing the spin then what was? What was causing the rounds to behave the way they were? I mean, if it were just a matter of weight and no spin was involved then wouldn't a heavier paintball show the same accuracy as the FN303 rounds since we are seeing these results with the lighter rounds? These results would seem to lead to the conclusion that there are forces acting on the ball that are greater than the vortexes we see on a round ball thus pushing the lighter round further off course but not affecting the heavier round.

It sounds to me like there may be something else at work here. hmmmmmmmm.

The rounds are spun because they have fins on them. If you search for Musikman's thread on his FN303 in Paintball talk you will see pictures of the rounds.

Heavier paintballs are more accurate. Same vortex force will have less effect on a heavier round. F=MA.

Ahhhhh...I have seen the rounds and saw the fins but did not know they were there to impart the spin.

I also realize that a heavier round will be less resistant to the force the vortex applies to it thus being more accurate.

So does this mean that when the lighter weight round was spun using the same fins (again, assuming that the round we are talking about here is identical in shape and form as the FN303 rounds and the only difference is weight) that this spin is what caused its erratic behavior?

Not trying to sound stupid...just trying to get my head around why this design change in a similarly weighted paintball would make its accuracy WORSE than when it was perfectly round. I can understand its accuracy not being as good as a heavier round, but I would still think there would be an improvement in accuracy over a round ball of equal weight. I would have never thought it would actually get worse. (BTW, F=MA is that Force=Mass * Acceleration? If so I'm amazed something from my HS physics class actually took! LOL)

Right. The FN303 rounds are 8 grams because they couldn't get the performance they wanted from lighter rounds. My question is was there any difference in performance and if so, how much? From what I'm reading here it sounded like they were worse but I don't know if these rounds were the exact same as the FN303 rounds but lighter or just "close proximities" to the FN303 rounds.

Right. The FN303 rounds are 8 grams because they couldn't get the performance they wanted from lighter rounds. My question is was there any difference in performance and if so, how much? From what I'm reading here it sounded like they were worse but I don't know if these rounds were the exact same as the FN303 rounds but lighter or just "close proximities" to the FN303 rounds.

Are you questioning my ability to reverse engineer?

Only difference in these rounds is what they were made from and how they were weighted.

Possibly because without the weighted front the spin made them tumble.

I think the problem is the center of gravity. The nylon rounds are back heavy. You need to have the center of gravity forward enough so that in flight, the backend isn't trying to become the bottom. With the back end too heavy, the spin axis is offset from the center axis of the round and there is a lot of drag from the finned end = unstable flight.

How about trying to drill out the back of the rounds to move the center of gravity forward.

That would also explain why the front half of a "normal" FN303 round is where the majority of the weight is placed. I'm almost positive that the final weight was not calculated to get a certain impact, but was what was required to sufficiently balance the desired payload.

Simon, when you made those rounds, did you use the dimensions I gave you?

Yes and checked them against some stuff in NJ. You'd be amazed at what they have here!

Originally Posted by SlartyBartFast

I think the problem is the center of gravity. The nylon rounds are back heavy. You need to have the center of gravity forward enough so that in flight, the backend isn't trying to become the bottom. With the back end too heavy, the spin axis is offset from the center axis of the round and there is a lot of drag from the finned end = unstable flight.

How about trying to drill out the back of the rounds to move the center of gravity forward.

That would also explain why the front half of a "normal" FN303 round is where the majority of the weight is placed. I'm almost positive that the final weight was not calculated to get a certain impact, but was what was required to sufficiently balance the desired payload.

I agree.

My next plan was to make some with hollow fronts I could fill, and with hollow rear ends to move the weight around.

I think it's all about the COG, although from past conversations with Tom I believe with the FN303 the weight is also there for impact and accuracy.

Hello all I have just spent a good part of two days reading this thread and find it very interesting. The one thing that I always found unusual about Tom's testing is that they did not find any improvement in accuracy by spinning the paintball. Now from what I understand spinning the ball should cause a tornado like vortex to form behind the ball rather than the random vortex shedding which is occuring behind a non spinning paintball. I was very confused by the fact that imparting a spin with it's axis along the line of flight did not improve accuracy. Then I got to thinking and I believe I may have come up with something. Now this is only mind experiments and I have no testing to back anything up so I will state this as a hypothesis.

Tom said they spun the balls to 10 000 rpm and that is the same speed a flatline reaches according to Tippman. Now from my basic in my head calculations I found the circuference of the paintball to be about 2" remember this number I will becoming back to it.

Now the air is flowing over the paintball at 300 fps or 3600 inches per second. Now to try and overcome this flow pattern we would most likely need the horizontal motion of the air(The air moving over the paintball along the direction of the spin) to match this velocity or exceed it. So 2" circumference means the air travels 2" per rotation. So that means we need 1800 rotations per second or 108000 rpm which way exceeds that which tom imparted to the ball.

Now I accept that this is a rediculous speed but may it be possible that at a certain speed the rifling(which is the driving force for the tornado like trailing vortex) will overpower the random vortex shedding and we will have a much more accurate shot.

Basically why I am putting up for debate is the reason rifling the ball did not work for Tom was the ball just was not spinning fast enough?

Wow, nice job guys! I know don't fully understand everything, espcially those formulas, but I believe that I have comprehended enough to possibly contribute (plese tell me if I'm wrong). When I finally reached the end (3 days worth of reading) and got to nino_fs's post, I think you were on to something. I may be talking way over my head, but anyway, I remember reading that the vortex sheds every 4", which could be totally off since I coudn't find this when I went back to double check. With this, I figure that in order to improve accuracy, you'd need to rotate the paintball once per shed, and that would be 54,000rpm. Once a test has been done with a paintball rotating at that speed, I'll be satisfied that spinning will have no effect at all, unless I have no idea what I'm talking about.

There are only 10 types of people in this world,
Those who understand binary and those who don't.

Vortex shedding is the same thing your hand does when it moves through water, the whirlpools behind your hand are the vortices produced. This is the air flow and action that would happen around just about any sphere that is moving through air, or even water.

While spinning the paintball in a direction parallel (like a bullet does) to the path of the paintballs travel might even out the vortices produced, the rate tested is far above feasible considering the fragileness of a paintball.

Looking at 3000rpm’s, you have a rate that is 50 times a second. In the 15ft the ball travels in one second (at 300fps), you have three twists in every foot, or one every 4 inches. 1:4” ratio. That is far above what is produced in a rifle, with rates like 1:48”. Since exaggerated (by 12 times) rotation is not producing the wanted (or even expected) affect other items are in play. These items seem to trump basic ballistic bullet physics.

How they are (if directly correlated like WorrNemesis suggested) affected by the parallel motion of a paintball is another matter. Since spinning a bullet stabilizes it, that alone could simply cut down on parasitic drag, from no spin or slowly rotating bullet not being stable enough to cut a clean path through the air. Since the sphere shape of a ball is not the best choice for a projectile, trying to adapt the physics of a non sphere designed projectile to a paintball is most likely a lost cause, even if it has been a fun argument.

One thing possible missed is the affect of a rotating sphere to produce an exaggerated boundary layer, in affect acting like a larger object as it moves through the air. This could actually produce more inaccuracy, as you would have a light object with a now even larger air profile footprint.

Food for thought.

Josh

"If you build it they will run" - pbjosh
MM006610 bought new in '94. One owner.http://itspaintball.com For Pneu Ideas

Yes, it was a very fun argument. I was hoping that there way a possible way to increase accuracy in some way. I think I understand a bit better, although it's kind hard since I haven't taken physics yet in school. Thanks!

Looking at 3000rpm’s, you have a rate that is 50 times a second. In the 15ft the ball travels in one second (at 300fps), you have three twists in every foot, or one every 4 inches. 1:4” ratio. That is far above what is produced in a rifle, with rates like 1:48”. Since exaggerated (by 12 times) rotation is not producing the wanted (or even expected) affect other items are in play. These items seem to trump basic ballistic bullet physics.

Um... Josh? How does a paintball travel only 15 feet in one second if it's moving at 300 fps?

At 3000 rpm, you have one rotation every 0.02 seconds. At 300fps, that equates to one rotation every six feet, for a twist of about 1:72".

I believe you're right on the mark with respect to high spin rates and the fragility of paintballs. At some point the shell will separate and spray fill all over the place. Anyone out there have access to a force tester that can measure tension? Care to destructively separate two halves of a paintball for us?

I thought this post was dead but I guess I should check back more often. Now I do agree and have always thought that the speeds I am suggesting to rifle a paintball at are completely impossible simply because the paintball would shatter. I was just throwing out an idea as to why it seemed that rifling did nothing when it should have some effect of the flight of the projectile. Also I had never thought of how rifling may actually create a large air shell around the ball which might be cancelling out any benefit from the rifling.

I have no idea if my idea has any merrit as I am only half way through my fluid dynamics course and only starting on flow. I also doubt we will get into the topics of vortex sheading in this course anyway. I don't really know if the vortex shedding can be controlled but I would hazard a guess that it is not completely random and uncontrollable. What I would say is that it is so complex that science and engineering currently have no explanation for it other than it seems to be random. It would be interesting to try and produce the high reynolds number flows in a much slower flow of a more viscous fluid. And then by introducing dye and trying different things one might be able to see how they can attempt to control the vortex shedding.

Even if it was possible to force more shedding in one direction by a backspin that could be extremely useful since it might then be possible to reduce the drift of the ball.

But this is all speculation and has no scientific backing. I may get back into this topic in the following weeks trying to find something in the University Library. So any ideas please post as I will see what I can find out.

Multiply by 20 the spin and it seems that the difference between the rifle twist and Tom's paintball test is alot closer.

One thought about vortex shedding and randomness:

Due to the "Chaos Effect" the shedding will be in a random order due to affects (air condition before ball passes, any shape irregularities in the ball, manipulation of the Matrix) too small to measure. These almost average out, and end up producing the accuracy/inaccuracy we see.

To distruct the Paintball in a manner that would demonstrate the affects of spin we just need to get a varible speed dremel or other tool, and glue a paintball to the end!

I was wondering if anyone knew the names of some books that discuss this random vortex shedding phenomenon. I ask because I tried to find information at the library but was unable to find anything as the library is huge and I don't really know what to look under.

I don't believe its random. It's a function of reynolds number, and the path will change based on this... at low Re, no vortex shedding, then as it steps up, you get a zigzag pattern, and then, at high Re numbers that are useful to us, a helical pattern.

Also, I've been thinking about rifling again. It seems to me that it would have some serious advantages with regards to accuracy if done correctly.

Prerequisites would be a marker with a spiky breech pressure to try to deform the balls, and a fairly tight rifled barrel.

You'd get the uniform aerodynamic profile, due to the averaging out of seam position.

You get paint void, which would cause off-line weight distribution, spun out due to centrifugal force, effectively normalizing weight distribution of liquid fill about the barrel axis

I'm not suggesting that the vortex shedding effect wouldn't occur, but I think that you COULD eliminate some variables. I hear very good things about the hammerhead. I think its irresponsible to wave one's hand in the air and dismiss spheres as poor aerodynamically.

- Because of (random) vortex shedding, we see a "random walk" effect on the paintball over the course of it's trajectory, which is (in summary of this thread) the "mystery force" that accounts (in significant part) for the spread of the impact point over the shot distances typically found in paintball.

- The spherical shape of a paintball doesn't lend itself particularly well to what I'll call "bullet physics", because at the rotation speeds necessary to counter-act the "random walk" a paintball would disintegrate

Thanks to anybody who's still sleeping on this thread with one eye open!

I recently dove head first into a mechanical engineering program, driven in part by a desire to understand and eventually be able to create things like paintball guns, and recently discovered how deep the "rabbit hole" goes. This has been a reassuring read, but a bit overwhelming, considering the unbelievable depth of knowledge some of you have collected. I sometimes wonder if I have that kind of room in my head...

(By the way - I fully recognize that this thread has seen no activity in 3 months... so, feel free to mention it anyways, I don't know the policies about ressurecting dead threads here, but if anyone feels like typing a bit more, I'd appreciate it )

On the bright side, knowing why isn't necessarily any better than knowing it happens. Many sucessfull paintball guns have been built on love and pasion rather than design and technology. (Stroker, cocker, sterling)

And if you wanna know how bad "engineering" can be. Look at the PVI shocker. And the seven years of slow guns that followed it. ;-)

Just be prepared to do a lot of testing. And make sure the testing is repeatable. YOu also have the whole of AO behind you. Decades of shared experience is available to you.

To be an AGD supporter, one cannot be an AGD bigot. -Nero

Truth is a complex thing. One must govern by simplicity. -M. Mercier, special counsel to his Majesty for domestic matters. The Brotherhood of the Wolf

"You can't outrun Death forever, but you can make the bastard work for it."

- Because of (random) vortex shedding, we see a "random walk" effect on the paintball over the course of it's trajectory, which is (in summary of this thread) the "mystery force" that accounts (in significant part) for the spread of the impact point over the shot distances typically found in paintball.

That pretty much covers it.

Originally Posted by mysteryfish

- The spherical shape of a paintball doesn't lend itself particularly well to what I'll call "bullet physics", because at the rotation speeds necessary to counter-act the "random walk" a paintball would disintegrate

No. The paintball can stand the rotation. It just doesn't do any good.